This invention relates generally to communications and, more particularly, to wireless systems.
In a wireless multiple access communication system, high success rate of xe2x80x9ccall setupxe2x80x9d in the access state (versus the conversation state) is a critical performance requirement. For CDMA (code division multiple access) systems based on industry standards IS-95A or ANSI J-STD-008, the success rate of call setup can be relatively poor in areas characterized by rapidly fluctuating fading conditions.
In these systems, a mobile station monitors the received signal-to-noise ratios of pilot signals from a number of base stations. During call setup, the mobile station selects the base station with the strongest measured received pilot signal and attempts call setup communications with only the selected base station. (This is in contrast to xe2x80x9csoft hand-off,xe2x80x9d which occurs during the conversation state and in which the mobile station communicates with a plurality of base stations.) In call setup, the mobile station first provides an xe2x80x9caccess request messagexe2x80x9d to the selected base station and then listens to (i.e., decodes messages from) the paging, or reply, channel of the selected base station. However, in rapidly fluctuating fading conditions, the received signals from the selected base station are dynamically changing which can result in poor signal quality on the paging channel at the time a response to the access request is sent (in the form of a channel assignment message) by the base station. As a result, the mobile station may not detect the channel assignment message from the base station to complete a call setup (i.e., establish a connection so that the mobile station can enter a xe2x80x9cconversation statexe2x80x9d), thus requiring the mobile station to repeat the entire call setup process.
An alternative approach is to modify the call setup procedure so that the mobile station transmits an extended access request message to the selected base station that includes a list of pilot strength measurements. That is, the mobile station still communicates with a single base station during call setup, but the pilot measurements tell the selected, or primary, base station what other base stations might potentially become stronger than the primary base station (in terms of their pilot strength at the mobile receiver) in the near future. Once the CDMA infrastructure receives these measurements, and the identities of the corresponding base stations, an xe2x80x9cextended channel assignment messagexe2x80x9d is subsequently simulcast to these candidate base stations, each of which (including the primary) sends a copy of the channel assignment message over their respective paging channels to the mobile station. In this approach, while waiting for the channel assignment message, if the mobile station detects that the pilot of the initially-selected primary base station fades, the mobile station then performs a hard hand-off and selects another base station to communicate with from the above-mentioned list. This is referred to herein as an xe2x80x9cidle hand-offxe2x80x9d during call setup.
Unfortunately, even with this latter approach there is still a period of time, albeit small, in which a mobile station may perform the idle hand-off too late and miss the channel assignment message from one of the alternate base stations, thus causing the mobile station to try the call setup procedure again.
In accordance with the invention, xe2x80x9csoft accessxe2x80x9d is performed. In soft access, a wireless station decodes call setup communications from multiple base stations in parallel. As a result, message reception diversity is provided during the call setup phase, thereby improving the success rate of a call setup.
In an embodiment of the invention, it is assumed a mobile communications system employs the above-mentioned extended channel assignment messaging scheme. During the call setup phase, the mobile receiver employs diversity reception in monitoring the paging channel of multiple base stations, each selected as a function of the strength of the signal-to-noise ratios of their respective pilot tones.